The fish disease furunculosis derived its name from the characteristic lesions observed as furuncles formed on the surface of fish as a result of infection with Aeromonas salmonicida. This pathogen causes most severe losses in production farms of salmon and trout, and leads to the use of large amounts of antibiotics in closed and open waters for therapy of fununculosis. In order to develop efficient strategies to prevent A. salmonicida outbreaks, it is essential to know the main mechanisms of pathogenicity of A. salmonicida. 
Several potential virulence factors of A. salmonicida have been described thus far. They include the surface array-layer protein, the hemolysins ASH1, ASH3, ASH4, H-lysin, salmolysin, the serine protease AspA and the Glycerophospholipid:Cholesterol Acyltransferase (GCAT) complexed with lipopolysaccharide (LPS). While there are many reports on potential virulence factors of A. salmonicida, in particular hemolysins, little is known about their activity and their role in pathogenesis. Many of them seem not to play a primary role in pathogenesis, since deletion mutants of GCAT and aspA genes showed neither of them to be essential for acute A. salmonicida-induced furunculosis. AspA however is essential for pro-GCAT processing in broth cultures and might also be involved in activation of other secreted enzymes or toxins.
Various attempts have been made to develop vaccines to prevent A. salmonicida infections mainly on the basis of killed cells (bacterins). Current vaccines achieve some level of protection. However, the nature of the antigens in efficient vaccines is not well defined. Significant differences of protein patterns are seen in cultures of A. salmonicida grown in vivo by an intraperitoneal implant technique in rainbow trout compared to cultures grown in vitro in culture medium. Such differences are thought to be the reasons of variable efficacy of former furunculosis vaccines due to a lack of appropriate antigens in certain vaccine preparations.
Several pathogenic bacteria use ADP-ribosylation as a key mechanism to modify properties of host cell proteins, thus to modulate their function and induce disease. Hence ADP-ribosylation of eukaryotic regulatory proteins is the underlying pathogenic mechanism of a heterogeneous family of bacterial protein toxins. ADP-ribosylating toxins are broadly distributed among highly pathogenic bacteria and are the primary cause of various severe human diseases such as diphtheria, cholera and pertussis. Among them, the ADP-ribosyltransferase toxin called exoenzyme S (ExoS) of Pseudomonas aeruginosa is one of the most prominent representatives. It is secreted via a type III-dependent secretion mechanism. Type II secretion systems generally have the potential to recognize receptors on target cells, induce biosynthesis of the corresponding toxins, and finally inject these bacterial toxins directly into the host cells without secretion to the medium. Recently, it was shown that ExoS is a bifunctional toxin containing an N-terminal part resembling the Yersiniae YopE toxin which catalyses rho-dependent actin depolymerisation, and a C-terminal ADP-ribosylating domain. Unique to most bacterial toxins, the ADP-ribosylating toxin ExoS does not have a rigid target protein specificity and ribosylates a number of target proteins including IgG3, apolipoprotein A-I, vimentin and several members of the Ras superfamily. Intracellular expression of the amino-terminal domain of ExoS elicits the disruption of actin, while expression of the carboxyl-terminal domain of ExoS possesses FAS (factor activating exoenzyme S)-dependent ADP-ribosyltransferase activity and is cytotoxic to eukaryotic cells. FAS is a member of the 14-3-3 family of proteins that regulate the activity of several eukaryotic enzymes. Prior to this invention, no analogues to ExoS have been found in bacteria other than P. aeruginosa. 